Abstract
Fire in tropical forests increases tree mortality, degrades forest structure and reduces carbon stocks. Currently, there are large gaps in understanding how fire affects understory forest structure and composition, interactions with fire recurrence, and long-term impacts. Understanding these changes is critical to evaluate the present and future response of tropical forests to fire. We studied post-fire changes in understory regeneration in forests in Mato Grosso State, southern Amazonia, Brazil, aiming to answer the following questions: (i) does forest structure (basal area) and tree community composition vary with fire frequency and time since the last fire? (ii) does the response differ among strata (e.g., sapling, larger trees)? (iii) are changes in diversity associated with changes in forest structure? We surveyed trees and lianas in previously structurally intact forests that underwent selective logging, followed by different fire histories, including 5 and 16 years after once-burned, 5 years after three times burned, and unburned (control). Overall, species composition (abundance, richness and number of families) and diversity were highest for the unburned treatment and lowest for the recurrent burned areas. Fire frequency negatively affected plant structure and basal area; plants from the most frequently burned areas had more than 50% less basal area than unburned sites. Richness was positively related to basal area in the three times burned sites and in the sixteen years regenerating site for all strata. Our results demonstrate the negative influence of frequent fires on both the composition and structure of small trees in Amazonian forest. These changes to the cohort of small-sized trees may persist and have long-term impacts on forest structure, affecting the capacity and direction of forest recovery. With wildfire widespread across the region and increasing in frequency, fire may negatively affect tree diversity in remaining selectively logged forests and affect regional carbon cycling with consequences for the global vegetation carbon sink.
Highlights
Logging in the Brazilian Amazon damaged 10,000−20,000 km2 y−1 of tropical forests in the early 2000’s and understory fires burned ca. 85,000 km2 of standing forest from 1999 to 2010 (Asner et al, 2005; Morton et al, 2013)
We addressed the following questions: (i) do changes in vegetation structure and community composition vary with fire frequency and time since the last fire? (ii) does the response differ among strata? (iii) are these changes in forest composition associated with changes in forest structure?
Changes in tree basal area due to fire was a strong predictor of changes in species richness, i.e., the loss by fire of large trees which contribute most to basal area affects composition of understory species
Summary
Logging in the Brazilian Amazon damaged 10,000−20,000 km y−1 of tropical forests in the early 2000’s and understory fires burned ca. 85,000 km of standing forest from 1999 to 2010 (Asner et al, 2005; Morton et al, 2013). Logging in the Brazilian Amazon damaged 10,000−20,000 km y−1 of tropical forests in the early 2000’s and understory fires burned ca. In the Amazon Basin, fires have been associated with roads, pastures, agriculture, and logging (Feldpausch et al, 2004; Alencar et al, 2006; Cochrane and Barber, 2009). Forest fires in the Amazon Basin are one of the most important drivers of tropical forest degradation, especially recently. In the Brazilian Amazon, fire effects on tropical forests were first studied in the 1980’s due to concerns of land clearing from wildfires (Uhl et al, 1981, 1982). If plant communities were adapted to past fires in Amazon forests, current burning would impact these differently (Brando et al, 2012). Fire impacts vary according to vegetation composition and structure, and with fire regime (intensity, frequency, rate of spread: Silva et al, 2005)
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